Telephone system



July 28, 1942. B. F, LEwls ETAL' TEDEPHONE SYSTEM Filed March 22, 1940 5 Sheets-Sheet l Vl Mim Qu Qu NG M W m w Gm E m n wv WH v. B

g, /NVENTORSAALUNDSTROM July 28, 1942. la. F. LEWIS ET Al.

TELEPHONE SYSTEM 5V sheets-sheet 2 Filed March 22, 1940 f.' 5w/s NVE/Vm AALUNDSTROM ATTORNEY 1940 5 Sheets-sheet 3 TELEPHONE SYSTEM E. F. LEWIS ET AL Filed March 22,

July 28, 1942.

July 22, w42. B. F. Lew@ Em, 2,291,244

TELEPHONE S YS TEM Filed March 22, 1940 5 Sheets-Sheet 4 B. FLEW/S NVENTORSAALUNDSTROM A TTOR/VEY July 28, 1942. B, F, LEWIS ETAL 2,291,244

TELEPHONE SYSTEM Filed March 22, 1940 5 Sheets-Sheet 5 E Ew/s NVENTORS AALUNDSTROM Patented July 28, r1942 f UNITED STATES PATENT OFFICE TELEPHONE SYSTEM Application March 22, 1940, Serial No. 325,344

14 Claims.

This invention relates to telephone systems and particularly to those in which automatic switching devices are used to establish conversational connections.

The objects or the invention are to facilitate the transmission of designations over telephone lines and particularly from subscribers stations to the central office; to safeguard against false operations with respect to the designation transmission; to insure adequate control over all functions of the equipment in the central oiice, including the control of release of the common equipment; to maintain good transmission conditions during the sending of designations; and otherwise to obtain improvements in systems of this character.

Systems have been proposed heretofore in which codes of alternating currents of diiferent frequencies, instead of the more usual direct current impulses, are transmitted over the line from the subscribers station to set up the desired designations in common central oiiice register senders. It has also been the common practice in systems using the direct-current-impulse method of sending designations to rely on the presence of the direct current flowing from the central oiiice sender over-the subscribers line loop to hold the sender, thereby permitting the immediate release of the sender at any moment the subscriber replaces his receiver. If, however, this method of supplying direct current to the subscribers line to hold the sender is used in a system designed to transmit the designations by frequency codes, as above noted, there is the possibility that sounds entering the microphone may modulate the direct current and produce frequencies that cause false registrations in the central ofIice sender.

According to a feature of the present invention these difficulties are obviated by vmeans of a system in which the direct current is removed from the subscribers line following the seizure of the sender in the central oflice, and in which the sender is held independently of the line during the period of transmitting the designation currents to the sender. If the call proceeds normally, the sender receives and registers the full designation, controls the selector switches accordingly, and releases itself. However, if the calling party abandons the call before the sender has progressed to the stage where it is capable of releasing itself in the normal manner, a special frequency code is transmitted from the substation to the central oice. The sender receives this special code, recognizes it as a signal that the 55 subscriber has replaced his receiver, and proceeds to release at once. This special frequency code is produced automatically by the substation transmitter as a result of replacing the telephone on its stand. The frequency transmitter may comprise a plurality of vibratory reeds for generating currents of corresponding frequencies, and the act of replacing the telephone causes the plucking of one or more of these reeds.

Another feature of the invention is a system of the character above described in which the replacement of the telephone plucks one or more reeds to send the special release code and also initiates the operation of a time delay mechanism. At the end of a predetermined interval, during which the special signal is being sent from the reed generator over the line, the delay mechanism opens the line, thereby disconnecting the generator and Arestoring the substation to its normal open-circuit condition.

A further feature is a system of the character described in which the resistance of the transmission circuit is held within desired limits while the designation is being sent. As above noted, the transmission circuit is dry during the sending period; that is to say no current flows in the circuit other than the alternating currents generated by the subscribers transmitter. But it has been found that the numerous relay and switch contacts in the transmission circuit present a much greater resistance to the alternating current signals when the circuit is dry than they do if the circuit is energized by the flow of direct current therein during the signal transmission. Not only is the resistance higher, but it is subject to Wide variations, which result in objectionable distortion of the signals. To overcome this objection, a source of alternating current of a relatively high frequency, above the audible range, is superimposed upon the circuit While the designation signal currents are being sent. This superimposed current does not interfere with the signal currents since its frequency may be chosenl at a value, say 30 kilocycles, which lies far beyond the range of signal frequencies. And, although relatively low in intensity, this auxiliary current flowing in the transmission circuit has the remarkable effect of maintaining the resistance of the circuit at a value comparable with that obtained when direct current is flowing and also the effect of preventing resistance variations. The intensity of the auxiliary current that is necessary to accomplish these results is low enough so that no interference is caused in the operation of other message or signaling channels.

These and other features of the invention will be described more fully in the detailed specification and will also be set forth in the appended claims.

In the drawing accompanying the specification, Figs. 1 to 5, when arranged in the order illustrated in Fig. 6, disclose, partly in diagrammatic form and partly in detail, an automatic telephone exchange system embodying the features of the present invention.

Fig. 1 illustrates the details of a subscribers line equipped with a transmitter for sending telephone designations over the line in the form ofV alternating currents of diierent frequencies. 'I-'his figure also illustrates diagrammatically the automatic switches in the central office which serve to establish telephone connections between subscribers lines, together with certain relays and control equipment associated with these switches. Furthermore, this figure illustrates the link switchesl which serve to extend a calling subscribers line to one of the common central oice register senders and A Figs. 2, 3, 4 and 5 disclose one of the register senders. Fig. 2 shows the controlling relays of the sender; Fig. 3 shows means Vforcontrolling the application of dial tone current to the calling line together with equipment for receiving the alternating current signals transmitted from the subscribers station; Fig. 4 discloses the transfer relays for transferring the designationY signals to the register relays; and Fig. 5 discloses the register relays on which the designations are registered.

Substation equipment ing the reeds. The mechanism illustrated i'n theV drawings comprises a plurality ofcode bars |02', one for each reed, and agroupof designation' keys |03. If, for example,`the designation characters are represented by codes of two frequencies, each of the keys may be provided with means for depressing two of the code bars |02. 4Each pair of code bars when depressed causes the plucking 'of the corresponding pair of vibrating reeds |00.-V A common bar |04 operates to shift the contact spring |05 vfrom out of engagement with contact |06 and into engagement with contact |01, whenever any one of the code bars |02 is depressed. Contacts |05, |06 and |01 serve toV disconnect the subscribers telephone transmitter |08 from the line and to substitute the generating coil |0| for the transmission of the code signals.

There may be as many keys in the set |03 as are needed for transmitting the various numeral and letter characters usedfor telephone designations. Also the number of frequencies comprising the code for each character may be varied if desired. Also it is obvious that the physical structure of the transmittermay assume many different forms. For example, the reedplucking transmitter disclosed in the patent to Mallina, No. 2,147,710 of February 21,4 1939, would in general be suitablefor Ause in the present system.

Since the central oihce battery'is disconnected from the line during designation transmission, it is necessary, as hereinbefore explained, to provide some means whereby the premature replacement of the telephone by the subscriber transmits a signal over the line which will effect the immediate release of the sender. This means comprises a lever |09, which is depressed by the cradle plunger ||0 when the subscriber replaces his telephone handset in the cradle of the supporting stand. The lever |09 depresses a single one of the code bars to pluck the corresponding single reed, thereby transmitting a special code of o'ne frequency to the central oiiice. At the same time the downward movement of the lever |09 tensions the spring which is strong enough to rotate. the toothed lever ||2 about its pivot ||3 and to shift the contact spring IIS to its alternate position against the tension of the weaker spring |52. The toothed sector of the lever ||2, however, meshes with the pinion attached to the balance wheel ||4, which in turn meshes with the fan governor ||5. The resistance thusoiered by the fan to the tension of the spring introduces an interval of delay between the downward movement of the plunger ||0 and the rotation of the lever ||2 suciently to move the contact spring |6 out of engagement with spring The opening of contact springs ||6 and ||1 disconnects the generator coil |0| from the line, thus-terminating thetimed transmission of the special signal current. The contact springs |50 and |5|., which are also shifted under control of spring` H6, are in the receiver circuit and may be adjusted to open slightly before contacts ||6 and. ||1 open. This avoids acoustic disturbances in the receiver when the direct current circuit is opened.

Central olce equipment At the central oice the subscribers line |20 terminates in a line iinder. switchV F, which is permanently associated through a link circuit with a first or district selectorswitch S. Selector S has access through succeeding switches, such as an ciice selector S-,-|, an incoming selector S-Z, and a final selector. S-S, to called subscribers lines. The line finder-district link circuit is automaticallyv associated withan idle register sender by means ofa link, circuit |2| comprising switches S-4 and S-5.

Although the inventionis not limited to a system using any particular-type of selector switch, it may be assumed that the. switches disclosed in Fig. 1 are of the well-knownpanel type, such as those disclosedv in numerous prior art patents including ther patent to Stearn et al. 1,395,977 of November'l, 1921.

The register sender shown in Figs. 2` to 5 includes, in addition to controling sequence switches 229 and 234 and-numerous circuit controlling relays, a, receiving. circuit for receiving the alternating currentV signals and` for translating them into registrations on the relay registers, which in turn serve to control the Aselective operation of the switches S, S-|, S-2 and S-3 to establish the desired conversational connections. This alternating current receivingcircuit includes a hybridcoil 300, which is coupled through a low-pass lter 30| andan amplier 302-to a common input circuit308rfor a series of-ve band-pass-lters 303, 304, 305, 305 and 301; These filters correspond respectively to the five reedsl 00 of the subscribers transmitter, each filter serving -to pass-the frequency produced by the corresponding reedand toexclude all other signaling frequencies. The output circuits of the filters 303 to 301 are connected respectively to detectors 309, 3|0, 3H, 3|2 and 3|3, and the output circuits of these detectors are connected respectively to the operating windings of transfer relays 400, 40|, 402, 403 and 404. The transfer relays 400 to 404 serve to operate the register relays shown in Fig. 5. vThe i'lrst three registers 500, 50| and 502 serve to register the three code letters of the oice designation; andl the next four registers, of which only Aone is illustrated, namely the units register 503, serve to register the numerical portion of the telephone designation. 'Ihe transfer relays during the course of the designation transmission are associated successively with these registers and serve to operate the relays of each register in accordance with the corresponding character received over the subscribers line. After the designation has been registered on the relay registers shown in Fig. 5, the sender proceeds to control the selective operation of the switches in accordance with these registrations.

For the sake of simplicity much cf the controlling equipment of the sender has been omitted. For a better understanding of the manner in which senders of this general type operate in the performance of their switch controlling functions reference is made to the prior art pat ents disclosing these senders in detail, and in particular reference is made to the above-noted patent to Stearn et al., No. 1,395,977; to the Raymond et al. Patent No. 1,862,549 of June 14, 1932; and to the Stearn Patent No. 1,505,171 of August 19, 1924.

Detailed description of operation 'I'he operation of the system will now be described in detail. For this purpose it will be assumed that the subscriber of line |20 wishes to converse with the subscriber of line |22 and that the designation of the called line is CHE-3567.

Initiation of Call To initiate the call the subscriber removes his telephone, which may comprise a handset including the transmitter |08 and receiver |23 in a single structure, from the cradle in the supporting stand. The removal of the weight of the telephone from the button |24 permits the plunger and the lever |09 to move upwardly. The upward movement of the lever |09 releases the depressed code bar |25 and cri-normal bar |00. The bar |04 opens` contacts |05 and |01 and closes contacts and |00. The bar |09 also releases the tension on spring permitting the lever ||2 and contact springs HB and |50 to be returned by the spring |52 to the position shown in the drawings, thereby closing first contacts ||0 and ||1 and then contacts |50 and |5|. Contacts |05 and |06 and contacts H5 and ||1 close the line loop, and contacts |50 and |5| close the receiver circuit. The closed line loop may be traced from conductor |21, through coil |23, transmitter |08, conductor |29, closed contacts |06 and |05, closed contacts |I1 and H0 to the other line conductor |30. The closure of this loop circuit initiates the operation of the line finder switch F in the usual manner, the subscribers line is seized by said nder, the link circuit 12| seizes the line finder-district circuit and also an idle register sender, and the calling line is extended through to the seized sender.

During the extension of the calling line through to the sender the sequence switch |33 of the district selector moves into position 3, and the controlling sequence switch of the link |2| advances to a position for closing the sleeve or control conductor from the district selector through the link to the sender. The path over this control conductor may be traced from battery through the Winding of relay |34, contacts of spring |31 and in parallel from battery through the wind- `ing of relay |38 through contacts of spring |31,

conductor |39, brush of switch 5 0, winding of relay |40 in the link circuit, contacts of spring |4| controlled by the link-circuit sequence switch, brush of switch S-5, conductor 2| 0, back contacts of relay 2| l, back contacts of relay 212 through the middle and right windings of relay 2|3 to ground. Relays |34 and |40, being marginal, do not operate in this ecircuit. Relay EIS, however, operates and closes an obvious circuit for operating relay 2|4. Relay 2|4 operates relay 2|5, and the latter locks in a circuit through its own front contacts. Relay 2|5 applies ground to the off-normal or holding conductor 210 for the purpose of holding the various relays that are to be operated in the sender during the course of its progress in controlling the establish ment of the desired connection.

A circuit is now closed including a source of direct current, the line relay 200 of the register sender and the extended subscribers line. This circuit may be traced from battery through resistance normal contacts of relay 202, right winding cf line relay 200, normal contacts of relay 203, conductor 204 through the brushes of link switches S-5 and S-4, conductor |3|, contacts of spring |32 of the sequence switch |33 associated with the district selector S, normal contacts of relay |34, brush of finder switch F, conduct-or |21, thence through the closed loop as above traced to conductor through the finder switch F, contacts of relay |34, contacts of sequence switch spring |35, conductor |35 through the brushes of link switches S-ll and S-5, conductor 205, contacts of relay 203, left winding of relay 200, normal contacts of relay 202, resistance 206, normal contacts of relay 201 to ground. Relay 200 operates and closes a circuit from battery through the winding of relay 202, front contact of relay 200 to ground through the back contacts of relay 201. Relay 202 operates Iand locks in a circuit from battery through its winding and front contact through the back contact of relay 208 to the grounded holding conductor 2|0. Relay 202 in operating disconnects battery and ground from the windings of relay 200 and from the calling subscribers line. Relay 200 thereupon releases and closes an operating circuit for slow-release relays 2|1 and 2|8. r1`he circuit for these relays may be traced from battery through their windings in parallel thence through the front contact of relay 202, back contact of relay 200 to ground through the back contact of relay 201. Relay 2| 1 disconnects the grounded bridge and extends the transmisison circuit from the calling subscribers line through to the input circuit 2|9 of the hybrid coil 300.

During the interv-al that direct current is owing over the subscribers line following the seizure of the sender a test of the line is made to determine its relative resistance for the purpose of adjusting the intensity of the key tone current which is transmitted from the sender to the subscribers station as a signal to him'that he may proceed to send the desired,designation, The

'purpose for making this adjustment is to obtain a uniformity in the key tone for subscribers lines of varying lengths and resistances. The mech-anism for making the adjustment includes vacuum tube 314, a plurality of gas-filled tubes 315, 316, 311 and 318, and associated relaysV for connecting a Variable number of shunt resistances across the secondary winding of the dial tone transformer 3`19. Normally the grid' electrode of the vacuum tube 314 isfree from bias, and maximum current flows from battery 320 through resistance 321 and the anode circuit of the tube to ground. The flow ofV current through the resistance 321 lowers to a minimum value the voltage applied across the control gaps of the discharge tubes 315 to 318, and none of these tubes operates. When, however, the calling line 120 is extended to the sender and batteryv is applied thereto through the windings of relay 200, as above described, -a negative. potential is applied to the grid of tube 314, the value of which is somewhat proportional to the resistance of the subscribers line- This bias reduces the ow of current through resistance 321 and increases the voltage applied across the control gaps of tubes 315 to 318 by a corresponding amount. These latter tubes are arranged to ionize at voltages of different values, and these voltages are obtained by connecting the control gaps of the tubes to different tap points on the secondary winding of transformer 324. That is to say, tube 315 will ionize at a minimum voltage, tube 316 at a larger voltage, tube 311 at a still larger voltage, and so forth. Assume, therefore, that the resistance of the calling line 120 is such that the first two tubes 315 and 316 ionize. The circuit for the control gap of tube 315 may be traced from battery 320, resistance 321, resistance 322, the control gap of tube 315, right winding of relay 323 through a portion of the second-ary winding of transformer 324 to ground. The tube ionizes, and current now flows in the anode circuit which may be traced from battery 336, contact of relay 331, which operated when ground was applied to conductor 216, anode gap of the tube 315, right winding of relay 323 to ground through the secondary winding of the transformer 324. Relay 323 operates in this circuit and locks in a circuit from battery through its left winding and front contact, conductor 326, front contact of relay 200to ground at the back contact of relay 201. As soon as relay 202 operates following the operation of relay 200, the locking circuit of relay 323 is closed over conductor 326 through the front contact of relay 202 and the back contact of relay 208 to the grounded conductor 216. In a similar manner the tube 316 is ionized and relay 321 is operated and locked to the grounded conductor 326. When the central office battery is disconnected from the subscribers line, the bias on tube 314 is removed, and full current flows again through the resistance 321. The control gaps of the operated tubes 315 and 316 are now ineffective, but the tubes and relays are held until the control ground is l-ater removed from conductor 215. Relay 323 closes a shunt across the transformer 319 traceable from one terminal of the transformer winding over conductor 328, contacts of relay 323, conductor 329, resistance 330 to the other terminal of the transformer winding. Relay 321 closes a similar shunt circuit including resistance 331. rhe relay 332 having operated to connect the key tone source to the primary winding of transformer 329, tone current, controlled in intensity by the shunt resstances 330 and 331, is transmitted through a suitable filter 333 overthe transmission circuit to the subscribers station.

Subscriber sends called designation The calling subscriber hears the dial tone and proceeds to transmit the desired telephone designation CHE-3567 by depressing, one after the other, seven of the keys 103. The rst three of these keys bear the letters C, H and E, respectively, the numerical equivalents of which are 2, 4 and 3, respectively. The last four keys depressed bear the numerals 3, 5, 6 and 7, respectively. Each key depressed causes the simultaneous plucking of two reeds 100. These reeds vibrate in the magnetic field and generatecurrents of corresponding frequencies in the coil 101. These alternating currents are transmitted over the circuit to the hybrid coil 300-from whence they pass through a selecting filter 301 to amplier 302. Here the alternating currents are amplified and applied to the common input circuit 308 of the band-pass filters 303 to 301, inclusive. The common input circuit 308 is also connected to an enabler 334` which controls the operation of the detectors 309 to 313, inclusive. These alternating current receiving circuits, which receive the signal currents and utilize them for registering the telephone designation, are illustrated diagrammatically herein since they are not particularly concerned with the features of the present invention. These circuits may, however, be of any suitable type and in general may be the same as those disclosed in the patent of A. A. Lundstrom, No. 2,237,742 of April 8, 1941.

When, therefore, the calling subscriber depresses the key, such as key |42, corresponding to the letter C, two of the code bars 125 and 143 are actuated to pluck the corresponding reeds 144 and 145. The common bar 104 isf depressed by the actuated code bars and remains depressed until the subscriber releases the actuated key 142. The bar 104 in its depressed position shifts the contact spring 105 to complete the transmission circuit for transmitting currentsv offrequencies corresponding to reeds 144 and 145 over the line to the receiving mechanism in the central office sender. This transmission circuit may be traced from the input circuit of the hybrid coil 300 over conductor 200,- left front contacts of relay 211, condenser 221, contacts of relay 203, conductor 205 through the link 121, conductor 136, contacts of spring 135, normal contacts of relay 134, brush of finder F, conductor 130, contacts 116 and 111, contacts 105 and 101, through the generating coil 101, conductor 121, brush of finder F, contacts of relay 134, contacts of spring 132, conductor 131, through the link 121, conductor 204, contacts of relay 203, condenser 222, rightr front contacts of relay 211 and conductor 223 to the input circuit of the hybrid coil 300. If it is assumed that the frequencies of the ve reeds 100. are, beginning with reed 144, fa, fb, fc, fd andV f, the actuation of key 142 causes currents of frequencies ftA and fe to be transmitted simultaneously over the circuit above traced to the hybrid coil.300. Coil 300 passes these currents to the conjugate branch including the filter 301, and they are amplified by the device 302 from whence they are applied' to the common input circuitl 308. These currents are then selected by the filters 303' and' 301, respectively, and applied to the detectors 309 and 313. Detector 309 responds toA current of frequency fa, and operating current flows through the winding of translator relay 400. Similarly the detector 3 I3 responds to current of frequency fe, and operating current flows through the winding of relay 404. Relays 400 and 404 lock through their windings and contacts, conductor 405, closed contacts of relay 406 to the grounded olf-normal conductor 2|6. Relay 406, it will be noted, operates as soon as ground potential is applied to conductor 2|6, the operating circuit of relay 406 extending through the normal contact of relay 401. As long as the transfer relays 400 to 404 are all in their normal positions, the grounded conductor 2|6 is extended through normal contacts of these relays to conductor 408. With ground connected to conductor 2| 6, condenser 4|| is charged, and the armature of relay 401 is held against the left-hand contact by the current that flows through the lower winding. When Vone of the relays, 400 to 404, operates, the ground through resistance 4|0 is removed. The current through the lower winding of relay 401 reverses and tends to move the armature to the righthand contact. However, the discharging current of condenser 4| I flowing through the upper winding tends to hold the armature on the left-hand Contact, and thus the movement of the relay armature from left to right is delayed until the discharge current of the condenser becomes small. After an interval measured by this discharging time of condenser 4|| the relay 401 operates and releases relay 406.

While the relay 401 is measuring the time interval above mentioned, transfer relays 400 and 404 are operated and locked in the circuits traced.

Relays 400 and 404 close a circuit traceable from the grounded conductor ZIE, through the operated contacts of relay 400, conductor 4|2, closed contacts of relay 404, conductor 4|3 through the closed contacts of switching relay 504, winding of register relay 505 to battery. It should be explained that relay 504 is in an operated condition at this time over a circuit traceable from battery through its winding, normal contacts of relay 500 thence over conductor 501 `which was grounded following the seizure of the sender. Register relay 505 operates and prepares a locking circuit traceable from battery through its right winding and contact, winding of relay 506 thence through the back contacts of relays 503, 500, 5|0 in series to conductor 5H. As soon as relay 401 completes the measurement of the time interval it releases relay 406, which extends the register locking circuit through its make contact to grounded conductor 2| 6 before ground is removed from conductor 405. Hence the register relay 505 locks over the circuit above traced through conductor 5|| and the back contact of relay406 to conductor 2|6 before ground is removed from conductor 405 to cause the release of transfer relays 400 and 404. The operated register 505 and relay 506, which operates in series therewith, remain energized in a permanent locking circuit which may be traced from battery through the right winding and contact of register relay 505, winding and front contact of relay 506 to the grounded control conductor 501. Relay 506 releases the switching relay 504 and closes an obvious operating circuit for the switching relay 508 corresponding to the next office register 50|. Thus the transfer relays are disconnected from the register 500 and associated with the register 50| ready for the registration of the next designation character. When the armature of relay 401 moves to the right-hand contact, it connects ground to lead 425. This reverses the current through the lower winding which then tends to move the armature to the left-hand contact. The charging current of the condenser, however, delays this action for a period suflicient to insure the release of relay 406, the operation of relays 500 and 508 and the release of relay 504. The release of relays 400 and 404 restores the ground to conductor 408 which holds the armature of relay 401 on the left hand contact until the next digit signal is received.

When the calling subscriber depresses the seoond key for the letter H, two reeds are plucked for generating and transmitting currents having frequencies fa and fc. These currents are selected by filters 303 and 305 resulting in the operation of transfer relays 400 and 402. During the timed operation of relay 401 these operated transfer relays close a register operating circuit from the grounded conductor 2|6, contacts of relay 400, conductor 4|4, contacts of relay 402, conductor 4|5, contacts of relay 508, left winding of relay 5I2 to battery. Register relay 5| 2 closes a circuit through its right winding and contact, winding of relay 5||3 thence through series contacts of relays 500 and 5| 0 to ground over conductor 5|| as soon as relay 406 releases. Relay 5|3 also operates in this circuit and closes a locking `circuit for register relay 5| 2 and for itself to the grounded control conductor 5|4. When relay 406 releases, transfer relays 400 and 402 release since, in the meantime, the enabler 334 has desensitized the -vacuum tube detectors. Relay 401 now times an 'interval and releases to reoperate relay 406.

In the manner above described the calling subscriber manipulates the keyboard to transmit the remaining letter E of the office designation and the numerical digits 3, 5, 6 and 7. The transmission of the letter E results in the operation of transfer relays 400 and 403, whereupon the register relays 5|5 and 5i6 are operated and locked. The transmission of the thousands, hundreds and tens digits, 3, 5 and 6, causes the operation of the proper register relays of the thousands, hundreds and tens registers, which have been omitted from the disclosure for the sake of simplicity. Finally the transmission of the units digit 7 causes the operation of transfer relays 402 and 403, and relays 5|1 and 5I8 of the units register 503 are energized and locked.

During the time the transmission circuit is closed between the substation of the calling line |20 and the hybrid coil 300, the central ofiice battery is disconnected from the subscribers line. As soon as relay 202 operates following the seizure of the sender the central oiiice battery is disconnected from the line and remains disconnected therefrom throughout the interval that the subscriber is transmitting the designation. Since the transmission circuit between the substation and the central oiiice sender involves numerous relay and switch contacts, it is found that the resistance of the transmission circuit to the flow of the alternating current signals is considerably greater and is subject'to wider variations than would be the case if direct current were flowing in the circuit during the signaling period. This disadvantage, however, is obviated by connecting a source of current 224 of relatively high frequency to the transmission circuit. The current from source 224, which may have a frequency of 30 kilocycles, iiows over the transmission circuit and has the effect of holding the resistance of the circuit to values comparable with those existing when direct current is flowing.

Connection established in .central ofice Following the registration of vthe vdesignation on the registers shown in Fig. the sender progresses through its `various stages of operation, and the district and ofce selector switches S and S-I are selectively positioned in accordance with .the oflice designation, and vthe 'incoming and final selectors S`2 and S-3 are positioned under the control of the .sender in accordance with the registered numerical designations. After selections have Vbeen completed the relay 2I2 is operated in the sender to remove the short circuit from the high resistance left winding .of relay 2I3. The inclusion of this resistance in the control circuit causes the release of marginal relay |38, which at this timeisoperated lin series with relay 2I3. Relay |38 releases and advances the district selector sequenceswitch |33 to some position, such as position I2. In position I2 of the district selector, ground potential 'is connected through sequence switch spring |46 .to the control conductor 2|.0 thus causing the ,release of relay 2| 3 in the sender. Relay 2I3 releases relay 2I4, and a circuit is .closed from battery through the winding of relay .2| I., front contact of relay 2I5 to ground .at the back contact of relay .2|4. Relay 2|I `operates and 'locks over conductor 2 I0 to ground .in the district selector. Relay 225 now operates in a circuit frombattery through its winding to ground at the contact of relay 2II. Relay VV225 .closes a circuit .through resistance 226 in shunt of the relay 2|.5 andrelay 2I5 releases. Ground is now maintained on the holding conductor 2I6 at the front contact of relay .2| I.

Following the advance of the .districtselector sequence switch to the proper talking position, relay 201 is operated in the sender. Relay 201 closes a circuit from battery through resistance 221, contact of relay 201, .front contact o'f relay 2| I, thence to control conductor Y2I`0 in parallel with the winding of relay 2I|. These parallel circuits increase the current ilowing in conductor 2I0 sufficiently to cause the operation of the marginal relay |40 .in the link circuit .I'2I Relay |40 causes the releaseof the link circuit |2I and the opening of the circuits extending to the sender. Thereupon relay 2|I releases and removes holding ground from the .conductor 2.I6, `resulting in the release of all operated relays `in the sender. Thus the registersender is restored to its normal position following the completion of its functions in the establishment .of the desired connection.

Abandonment of ycall It will be assumed'next that the calling 'subscriber abandons the call at sometime prior to the completion of the operation of the selective switches. In such a case it is, of course, .desirable to effect the immediate release of the register sender in order that it may be available for other calls. When the calling subscriber replaces his telephone to .abandon thecall before the connection is completed, .a special signal is transmitted to the .register .sender to notify it to release at once. If the abandonment occurs while .the designation is being sent, relays 2I3, 2'I4, 2I5, 202, 2I1 and 2I8 are in an operated condition.

The transmission to the sender of the special signal at this time is caused `by the depression `of plunger I I0 when the telephone engages -therbutton '|24 as it is replaced inthe cradle of the-supporting stand. The 'depression Yof the plunger II0 rotates the .lever 4|09 to .operate the single code bar |25 which plucks the reed |44 corresponding to frequency fr. Lever |09 also tensions the spring III for `causing the rotation of lever I'I2 Ato yshift the spring H6 after a delay interval caused Vby'theresistancethe fan II5 offers to the gear wheel I I4. The single frequency signal vtransmitted over the line is received by the sender to cause the operation ,of transfer relay 400 in the manner above described. Relay 400 closes a circuit from .the grounded conductor 216 through its front contact, conductor 4I6, back contacts yof relays 4I1 and 4I8, switch 4I9,

`back contact .of relay 420, back Contact of relay 404, conductor 42| A.through the winding of relay 'L208 to battery. VRelay 208 operates and locks through its own front contact to the grounded .holding Vconductor 2.|,6. Relay 208 opens the cir- .cuit of relay .2.02 .and connects battery and ground .through resistances 20| vand 20B through thewindings -of :relay 200 to the subscribers line circuit. The'purpose of connecting relay 200 to the line circuit at this time is to make .a test Vto determine whether the subscriber has replaced his telephone permanently or momentarily. Since it was assumed he abandoned the call, the loop is open, ,and relay 200 fails to operate. Therefore, relay 202 releases and opens the circuit of slow-release relays 2I1 and 218. When relay 2I8 releases, time 4measuring sequence switch 229 having advanced to the proper position, a circuit is closed for operating relay 230. This circuit may be traced Vfrombattery through the winding of relay'230, contacts ofspring 23| 4on the switch 229, Aconductor 232, back contacts A,of .relay 2| 8, back contacts .of relay '201, back ,contacts of relay 2I2 to the grounded conductor 2I6. Relay 230ilocks 'through its own contact directly to the grounded conductor 209. Relay 230, when operated, closes an obvious operating circuit for relay 201. Relay 230V also 'closes a short circuit around the right-winding of relay'2 I3. This short circuit reduces the resistance in series with relay |34 in the districtselector sufficiently to permit this relay to operate. Relay I'34 opens the talking conductors and Ain the well-known manner causes 4the vdistrict sequence switch |33 to advance, bringing about 'a restoration of the district selector,rif the same has been operated, and the release of the circuits to their normal condition. When the district selector sequence switch reaches position II, ground potential is connected to the controlconductor -2I0 through the contacts of spring |46. Relays 2I3 and 2I4 in the vsender release. 'Relay `2I1| operates as above described and connects battery through the resistance 221 and the front contact of relay 201 to the control conductor 2'I'0. The link circuit I2I also releases atthis time, and the sender is disconnected from the vline finder-district link. The sender now fully restores -to its normal condition ready for the next use.

Accidental manipulation of substation equipment vloop-following the roperation of relay 208-and the release of relay 202and before relays -2|-1 and 2I8 have time to release. The operating circuit for relay 200 may now be traced through the front contacts of relay 208 and thence to battery and ground through resistances 20| and 206. Relay 200 holds in this circuit following the reoperate tion of relay 202. Relay 200 in operating recloses the operating circuit of relay 202, and the latter relay closes a new holding circuit for relays 2I1 and 2|8. This holding circuit may be traced from battery through the windings of relays 2l'l and 2|8 in parallel, front contact of relay 202, front contact of relay 208 to ground through the back contact of relay 201. Thus relay 2| 8 is prevented from releasing, and relays 230 and 201 do not operate under these conditions. Relay 208 closes a circuit for sending the subscriber a special tone signal which he will recognize as an instruction that he should replace his telephone and initiate the call again. Current for the reorder tone flows from the source 228 through the front contacts of relay 208 thence through condensers 22| and 222 over conductors 204 and 205 and over the loop of the calling line to the subscribers receiver |23. When the calling subscriber replaces his telephone in response to the reorder tone, relay 200 releases and opens the holding circuit of relay 202. Relay 202 releases and causes the release of slow relays 2H and 2I8. Thereupon the sender proceeds as above described to restore to its normal condition.

It was assumed herein that the special release signal consisted of an alternating current of a single frequency. Either of the frequencies fa, fb or fc may be used singly, in which case the switch 4|9 in Fig. 4 is closed as illustrated, and switches 422 and 423 are open. If it is desired to use frequencies fd or fe alone, switch 423 should be closed and switches 419 and 422 opened. Also frequency fc may be used by closing the switch 422 and opening switches 459 and 423. Also other combinations may obviously be employed.

What is claimed is:

1. In a telephone system, a subscribers line having a substation, automatic switches, a sender common to said switches, means responsive to the closure of said line at the substation for seizing said sender and for establishing a connection between said line and said sender, a source of current included in the established connection for supplying current thereto, means for disconnecting said source from said established connection between the line and sender without releasing said sender, means for transmitting designations over said line to said sender, means in said sender for controlling said switches in accordance with the designations received, means responsive to the replacement of the subscribers telephone at said substation for transmitting a signal over said line, and means in the sender responsive to said signal for eiecting the release of said sender.

2. In a telephone system, a subscribers line having a substation, automatic switches, a sender common to said switches, means responsive to the clsure of said line at the substation for seizing said sender and for extending said line to said sender, a source for supplying current to said line, means for including said source in circuit with said line and for disconnecting it from Vsaid line, means independent of said source for holding said sender, means for transmitting designations over said line to said sender, means in said sender for controlling said switches in accordance with the designations received, means for releasing said sender following the completion of its switch-controlling functions, means at said substation for transmitting a release signal over said line, and means effective if said signal is received before the sender has completed its functions for causing the immediate release of said sender.

3. In a telephone system, a telephone line, automatic switches, a controlling mechanism for said switches, means responsive to the closure of said line for seizing said mechanism and for connecting said line thereto, a source of current in circuit with said line for energizing the same, means for disconnecting said source from the line, means for holding said mechanism after the disconnection of said source, means for sending telephone designations over said line, means in said mechanism for receiving and utilizing said designations to control said switches, means for sending a release signal over said line, and means responsive to said signal for effecting the release of said mechanism.

4. In a telephone system, a subscribers line, automatic switches, a transmitter at the substation of said line for transmitting telephone designa-tions over said line, a sender, means controlled over said line for connecting it to said sender and for holding the sender in a seized condition unaffected by the flow of current in said line during the transmission of designations,

-means in the sender for receiving and utilizing the transmitted designations to control said switches, and means controlled over said line for releasing the seized sender.

5. In a telephone system, a telephone line, automatic switches, a sender for controlling said switches, means responsive to the initiation of a call on said line for extending the line to said sender, a source for supplying current from said sender to said line when the latter is connected to the sender, means responsive to the current flowing from said source over said line for disconnecting said source from said line, means for sending telephone designations over said line to the sender after said source of current has been disconnected, means in said sender for receiving said designations and for utilizing them to control the selective operation of said switches, and means for transmitting a signal over said line t-o the sender to cause the release of said sender.

6. In a telephone system, a subscribers line, automatic switches, a transmitter at the substation of said line for transmitting over said line alternating currents of different frequencies representing a telephone designation, a sender, means responsive to the initiation of a call on said line for connecting it to said sender and for maintaining the sender in a seized condition unaiected by the ow of current in said line during the transmission of the telephone designation, means in the sender for receiving and utilizing the designation currents to control said automatic switches, and means controlled over said line in response to the replacement of the subscribers telephone for releasing the sender.

'7. In a telephone system, a subscribers line, automatic switches, a transmitter at the substation of said line for transmitting telephone ldesignations over said line, a sender, means responsive to the closure of said line for seizing said sender, a control circuit including said line established between the sender and said substation following the seizure of the sender, a source for causing direct current to flow over said control circuit, means controlled by the ilow of direct current in ,said control circuit for preparing the sender to receive said telephone designations, means for -holding said sender independently of the flow of direct current in said line, means in the sender for receiving and utilizing the -transmitted designations to control said switches, and means controlled over said line for releasing said sender.

8. In combination, a telephone line, automatic switches, a register sender, means for connecting said line to said register sender, a source of current Yincluded in the connection established between said `line andsender for supplying current to said line, a transmitter for transmitting telephone designations -oversaid line, means for disconnecting 4said source to prevent the iiow of current therefrom lover said line while said designations are being transmitted, means forholding said register sender while said source is disconnected, means in the register sender for receiving and utilizing lthe transmitted designations to control said switches,.and means for releasing said register sender.

9. VIn a telephone system, a telephone line, automatic switches, la transmitter at the substation of said linel for Atransmitting telephone designations rover said line, a sender, means controlled over said line for connecting it t0 said sender, releasing means forsaidsender unaifected by the condition of said line during the transmission Yof said telephone designations thereover, means in the sender for receiving and utilizing transmitted designations'to control said switches, means responsive tothe 'replacement of the subscribers telephone for sending a release signal over said line, means Jfor discontinuing the transmission of said release signal at the end of a predetermined interval following thereplacement of said telephone, and means responsive to said signal for effecting the release of said sender.

l0. In a telephone system, a telephone line,

'automatic switches, 'a transmitter 'at the substation of said line for transmitting telephone designations over said line, a sender, ,means controlled over `said 4line for connecting it to said sender, releasing `means for said sender unaffected'by the condition of said line Aduring the transmission o`f said telephone designations thereover, means in the sender for receiving and utilizing transmitted designations to control said switches, means responsive to the replacement of the subscribers telephone for opening said line to discontinue the calling condition thereof land for generating and transmitting a release signal over said line to said sender, a delay mechanism responsive to the replacement of said telephone for terminating after an interval the transmission of said release signal, and means responsive to said release signal for causing the release of said sender.

11. In a telephone system, a subscribers line, automatic switches, a transmitter at the substation of said line comprising a plurality of vibratory reeds having different frequencies, means controlled by the subscriber for plucking said reeds in different combinations to transmit currents over said line representing a telephone designation, a sender, means responsive to the closure of said line for connecting it to said sender, means in the sender responsive to the designation currents transmitted over said line by said transmitter for controlling the selective operation vof vsaid switches, means responsive to the `replacement of the vsubscribers telephone for plucking one of said reeds to transmit a release signal over said line, and means responsive to said signal for effecting the release of said sender.

12. In a telephone system, a subscribers line, automatic switches, a transmitter at the substation of said line comprising a plurality of vibratory reeds having different frequencies, means controlled by the subscriber -for plucking said reeds in different combinations to transmit currents over said line representing a telephone designation, a sender, means responsive to the closure of said line for connecting it to said sender, means in the sender responsive to the designation currents transmitted over said line by said transmitter for controlling the selective operation of said switches, a time delay mechanism associated with said transmitter, means responsive to the replacement of the subscribers telephone for initiating the operation of said delay mechanism and for plucking one Aof said reeds to transmit over the line a signal current of corresponding frequency, means in said sender responsive to said signal current for effecting the release of said sender, and means operated by said delay mechanism at the end of a predetermined interval for disconnecting said transmitter to terminate the sending of said'signal.

13. In a telephone system, a telephone line, automatic switches, a sender for controlling said switches, means responsive Vto the vclosure of said line for connectingjthe same to said sender, a control relay in said sender, a source of current connected to said ltelephone line through the winding of said relay, means responsive to the operation of said relay in a circuit including said telephone line for disconnecting said source of current from said line, means for transmitting telephone designations over said line to said sender after said sourceof current has been disconnected therefrom, and a'source for supplying alternating current to said line to control the impedance thereof during 'the transmission of said telephone designations.

14. In a telephone system, a telephone line, automatic switches,a sender for controlling said switches, means responsive to the closure of said line for connecting the same to said sender, a

Acontrol relay in said sender, a source of current connected to said telephone line through the winding of said relay, means responsive to the operation of said relay in a circuit including said telephone line for disconnecting said source of current from said line, transmitting means for sending over said line alternating currents of different frequencies representing 'telephone designations, and means for controlling the resistance of said line during the transmission of said alternating currents, said means comprising a source for supplying alternating current to said line of a frequency beyond the range of frequencies employed for representing said telephone designations.

BENJAMIN F. LEWIS.

ALEXIS A. LUNDSTROM. 

